DC/DC power converters are used in a variety of power systems, such as fuel cell/battery hybrid power propulsion systems currently in use in operational buses. The DC/DC power converters can either be boost type, if the desired output voltage is higher than the available input voltage, or it can be buck type if the desired output voltage is lower than the available input voltage. DC/DC converters may also be bidirectional, allowing power flow in both directions, or they may be unilateral, allowing power flow in only one direction.
The typical prior art DC/DC converter associated with a fuel cell stack is controlled by a system, which is usually digital, that adequately controls the converter input current (which is the output current of the DC power source e.g., fuel cell stack) and the converter output current and voltage. However, prior strategies have not been effective for managing DC power source output voltage constraints. Fuel cells have a monotonic voltage/current performance curve relationship. At low output power, the voltage of each cell can become sufficiently high to cause corrosion of the cathode and anode catalysts and the carbon catalyst supports. This corrosion causes permanent decay in the performance of the fuel cell.
Heretofore, prevention of fuel cell performance decay, as a result of catalyst and support corrosion at high cell voltages, has typically been prevented by means of a voltage limiting device (VLD) such as an auxiliary resistive load, which is caused to effectively be a variable load by switching it in and out of the circuit by means of an electronic switch controlled by a pulse width modulation (PWM) signal.
Referring to FIG. 1, a DC/DC converter 10, in this case unidirectional, and most likely a boost converter, is shown receiving power over a line 12 from a fuel cell stack 13, in a fuel cell power plant system 15 which provides power to a load 16. The load 16 may comprise an electric motor in an electric or hybrid vehicle, or may comprise any number of loads serviced by a stationary fuel cell power plant, such as a telephone exchange, a hospital, or a power distribution system requiring peak power assistance.
In FIG. 1, a resistive auxiliary load 29 is selectively switched in and out of the circuit by means of an electronic switch 30 controlled by a PWM signal on a line 32 provided by a VLD controller 34. Whenever the cell voltage on the line 12 exceeds a threshold, typically a few hundredths of a volt below the critical corrosion threshold, the VLD controller increases the duty cycle of the switch 30, lowering the average resistance to increase current and power output. The VLD controller will decrease the duty cycle by an increment whenever the fuel cell output voltage on the line 12 decreases below a lower, safe voltage. The auxiliary load 29, in dissipating any amount of power required to retain the safe cell voltage, creates heat that must be accommodated within the confines of the apparatus involved. The VLD controller is typically separate and apart from the fuel cell power plant controller as well as the DC/DC converter controller.
The inputs to the controller of the DC/DC converter are provided on a plurality of signal lines 19-21 as illustrated in FIG. 1. The limit signal for the fuel cell stack output current (DC/DC converter input current) Icel LIM is on line 19. The converter output current limit signal, Iout LIM is on line 20. The desired converter output voltage command, Vout CMND is on line 21.
Referring to FIG. 2, the prior art control strategy, for the controller 11 of the DC/DC converter 10 of FIG. 1, is reached through an entry point 37 and a first test 38 determines whether the output voltage equals or is greater than the commanded output voltage, Vout CMND. If it is not, a negative result of test 38 reaches a test 40 to determine if the fuel cell output current, Icel, exceeds the corresponding limit, Icel LIM. As used herein, the term “exceeds” means, with respect to the value of a parameter, that the value has reached a less favorable side of a limit or threshold thereof. If it does, then an affirmative result of test 40 reaches a step 42 which causes a duty cycle signal for the DC/DC converter to be decreased. These currents are described more fully concerning the present strategy with respect to FIG. 4, hereinafter.
If the fuel cell output current does not exceed a corresponding limit, a negative result of test 40 reaches a test 43 to determine if the DC/DC converter output current, Iout, exceeds a related limit. If the converter output current exceeds its limit, an affirmative result of test 43 will reach the step 42 to decrement the duty cycle, causing power to diminish. But if both the fuel cell output current and the converter output current are within limits, then negative results of tests 40 and 43 will reach a step 45 to increase the duty cycle. In other words, if the output currents are both in limit, then when the voltage output is less than the commanded voltage output as indicated in test 38, the duty cycle is increased at step 45 which will increase the output voltage of the DC/DC converter and cause an increase in output power.
If the voltage output of the DC/DC converter exceeds the output voltage command, a negative result of test 38 will reach the step 42 to decrease the duty cycle. This causes a decrease in the output voltage, and a decrease in the output power. Thus, the step 45 pushes the power (increases it) whereas the step 42 causes power to be diminished (not to be pushed). After either of the steps 42, 45, other routines are reverted to through a return point 48.